This invention generally relates to photoreactive compositions useful in a wide variety of photomechanical processes and, more particularly, to photoreactive compositions which find advantageous utility as coatings for presensitized lithographic plates. In this regard, an important aspect of the present invention is particularly directed to the conjoint use of a water soluble diazo resin layer together with an overlayer which imparts both increased photospeed (i.e. substantially reduced required exposure time) and photochromism so that a visually perceptible image is immediately produced upon exposure of the composition to light.
Lithographic plates having a photopolymerizable coating over a diazo resin are described in the literature. For example, U.S. Pat. No. 3,808,004 describes a presensitized lithographic plate having a diazo resin layer in overlying relation to a support plate and a cinnamoylated resin photopolymer in overlying relation to the diazo layer. Such plates are characterized by rapid exposure speeds (they are typically ten times as fast as emulsion-developed diazo resin plates and fifty to one hundred time as fast as emulsion-developed photopolymer plates), long running on presses (as long running as conventional photopolymer plates and longer running than conventional lacquered diazo resin plates), and are easily and rapidly developed. These plates, however, are not characterized by photochromism, an important property which, if present, would permit a user to visually inspect the same during imaging, thereby facilitating layout and composition therewith.
While the use of photochromic compounds, such as, for example, the spiropyran compounds, in photoreactive compositions is known, heretofore such use with diazo resins has been thought to be counterproductive since the energy absorbed by the spiropyran compound operated to suppress the occurrence of photoreaction of the diazo compound. U.S. Pat. No. 3,804,628, for example, described a photosensitive composition which includes a cinnamoyl group and an indolinobenzospiropyran derivative together with a sensitizer. The indolinobenzospiropyran derivative generates image coloration immediately upon exposure thereby facilitating photocomposition and enabling lithographic plates incorporating the same to have portions thereof sequentially imaged without encountering problems such as double exposure and blank printing.
In addressing themselves to the use of these photochromic compounds with diazo compounds, however, the patentees in U.S. Pat. No. 3,804,628 note that since the photosensitive wave length region of the diazo compounds almost coincides with the photosensitive wave length region of the spiropyran compounds, a severe reduction in sensitivity of the diazo compound is encountered when a diazo is used in conjunction with a spiropyran compound. As such, this patent demonstrates the heretofore commonly accepted belief that these photochromic compounds could not be used with water soluble diazo resins and still result in a photoreactive composition which exhibited a satisfactory photospeed.
In accordance with an important aspect of the present invention, it has been discovered that the conjoint use of photochromic compounds, such as the spiropyran compounds, and water soluble diazo resins can be accomplished where the photochromic compound is incorporated in a cinnamoylated or acrylated photopolymer overlayer on the diazo resin without reduction in the high photospeeds obtainable through the use of the diazo resin and photopolymer.
It is, therefore, an important object of the present invention to provide an improved photoreactive composition which exhibits photochromism and which finds advantageous utility as a photoreactive coating for presensitized lithographic plates and other photomechanical plates.
Another object of the present invention is to provide a high speed water soluble diazo resin and photopolymer composition which is characterized by an intense visible image immediately upon being exposed to light such as, for example, through a suitable stencil.
Another object of the present invention is to provide an improved high speed water soluble diazo resin based photoreactive composition which immediately produces a dark image when exposed to light as opposed to a lightened image of the type commonly associated with conventional bleaching dye image technology.
Another object of the present invention is to provide an improved photoreactive composition which includes a water soluble diazo resin on which a photochromic material has been applied together with a cinnamoylated or acrylated photopolymer resin, which composition is characterized by substantially increased photospeeds when compared to unlacquered diazo and prelacquered diazo photoreactive compositions and equivalent photospeeds when compared to diazo resin compositions to which only a cinnamoylated photopolymer has been applied.
Another object of the present invention is to provide an improved water soluble diazo resin/photopolymer lithographic plate wherein the resulting image produced during exposure is preserved with the use of clear developers and without requiring the use of pigmented or dye-based developing systems.
Another object of the present invention is to provide an improved presensitized lithographic plate having a polymer coating system which is relatively transparent, thereby permiting more simplified coating techniques to be used and avoiding coating and handling problems commonly associated with highly colored polymer coating systems.
Another object of the present invention is to provide an improved presensitized lithographic plate exhibiting photochromism and characterized by high photospeeds (reduced exposure times) and which is long-running on presses, as long running as conventional photopolymer plates and longer running than conventional lacquered diazo resin plates.
Another object of the present invention is to provide an improved presensitized lithographic plate characterized by increased ink attractivity and improved background cleanliness.
These and other objects of the present invention will be apparent from the following description of some preferred embodiments.
In accordance with the present invention, an improved high-speed photoreactive composition is provided which finds advantageous utility as the photoreactive coating in a presensitized lithographic plate. In particular, the photoreactive coating compositions of the present invention produce an intense visual image immediately upon exposure to light and comprise a light-sensitive, water soluble diazo resin layer on which an overlayer coating which includes a photochromic compound and a light sensitive polymer selected from the group cinnamoylated and acrylated photopolymer resins has been applied. Generally, the spiropyrans such as, for example, the indolinobenzospiropyran compounds are the preferred photochromic materials and the cinnamate esters and acrylic esters are the preferred light sensitive photopolymers.
I have found that the thinner the diazo film, the faster the exposure speed. Typically, amounts of photochromic compound of from 1.5 to about 5 weight percent, based on the weight of the photopolymer, provide effective color intensity, although greater amounts of photochromic compound can be employed in the overlayer if desired.
In instances wherein the photosensitive composition of the present invention is used in the manufacture of lithographic plates, such plates will have a backing or support member of aluminum, zinc, paper, plastic or equivalent sheet material. Typically, examples of plastic films or sheets will be composed of polyvinyl chloride, polyvinyl acetate, polyolefins and the like. Aluminum sheets of sufficiently low gauge so as to be readily flexed, however, are the preferred backing member. The plate is treated to provide a hydrophilic surface before application of the diazo resin; also, the diazo resin requires protection from direct contact with the metal. Preferably, the plate is then coated with a sub-layer or sub-base, many of which are commercially available. Suitable sub-bases, for example, include the phytic acid sub-base of U.S. Pat. No. 3,307,951, the melamine-formaldehyde condensation resin overcoated with a polyacrylamide, treated with zirconium acetate as in U.S. Pat. No. 3,073,723, the urea-formaldehyde sub-base of U.S. Pat. No. 3,136,636, the titanium ortho ester sub-base of U.S. Pat. No. 3,231,376, the silane-acrylic sub-base of U.S. Pat. No. 3,163,534, and the silicate sub-base of U.S. Pat. No. 2,714,066. In addition, other sub-bases are also satisfactory for many uses, including gelatin, polyacrylic acid and water-soluble salts thereof, polymethacrylic acid and water-soluble salts thereof, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, some titanates, modified resins of urea-formaldehyde and melamine-formaldehyde, polyvinyl alcohol, ferrocyanides or bichromates of sodium, potassium and ammonium, and the oxide of a metal forming the plate. Combinations of the foregoing materials are also suitable.
Any water-soluble diazo resin that is light sensitive may be used in the photoreactive composition of the present invention. Typical diazo resin sensitizers which are ideally suited for use in preparation of presensitized negative-working lithographic printing plates are described in U.S. Pat. Nos. 2,100,063, No. 2,667,415, No. 2,679,498 and No. 2,958,599. A preferred diazo resin of the negative-working type is prepared by condensing paraformaldehyde with p-diazo-diphenylamine sulfate as set forth in U.S. Pat. No. 2,100,063. The dispersion or solution of the sensitizer or light-sensitive diazo resin material may be applied to the prepared base, over the sub-base, by dipping, spraying, roller coating, brushing, or the like, all of which are conventional in this art.
Generally, any photochromic material which is reasonably transparent in the near ultraviolet region but which develops color upon irradiation with ultraviolet light may be employed as the photochromic compound in the photochromic/photopolymer overlayer. The spiropyrans represent a preferred class of photochromic compounds with the indolinobenzospiropyran compounds having been found to be especially suited for use as the photochromic material in the photochromic compound/photopolymer overlayer of the photoreactive compositions of the present invention. Illustrative materials which can be used as the photochromic constituents in the compositions of the present invention include: 1',3',3-trimethyl-6-nitrospiro-[2H-1-benzopyran-2,2'indoline],1,3,3-trimet hyl-'-nitro-8'-methoxyspiro(indoline-2,2'-benzopyran), 1,3,3,5tetramethyl-6'nitrospiro(indoline-2,2'benzopyran), 1,3,3-trimethyl-5-methoxy-6'nitrospiro(indoline-2,2'-benzopyran), 1,3,3-trimethyl-5-carboethoxy-6'-nitrospiro(indoline-2,2'-benzopyran), 1,3,3-trimethyl-5-chloro-6'-nitrospiro(indoline-2,2'-benzopyran), 1.beta.-carboxyethyl-3,3-dimethyl-6'nitrospiro(indoline-2,2'-benzopyran), 1-.gamma.-carboethyoxypropyl-3,3-diemethyl-6'-nitrospiro (indoline-2,2'-benzopyran), and, 1-.gamma.-cyanopropyl-3,3-dimethyl-6'-nitrospiro(indoline-2,2'-benzopyran) . Of the foregoing materials, the 1',3',3'-trimethyl-6-nitrospiro-[2H-1-benzopyran-2,2'indoline] compound has been found to be highly satisfactory for use with cinnamate ester and acrylic ester photopolymers in the practice of this invention.
The cinnamoylated resins which can be employed in the photochromic compound/photopolymer layer of photoreactive compositions embodying the present invention can be generally described as oligomers and polymers having recurring pendant groups corresponding to the following structural formula: ##STR1## wherein "X" is S, O, or N-R with R being H or lower alkyl, and the "AR" group is intended to include all structures exhibiting aromaticity, including carbon ring structures such as, for example, phenyl and diphenyl; fused ring structures such as, for example, derivatives of napthalene, anthracene and phenanthrene; and, heterocylic structures such as, for example, derivatives of pyridine and furan. In the preferred embodiment of this invention, "X" is O and the "AR" group is phenyl.
Illustrative examples of cinnamate ester polymers which can be employed with the photochromic compound in the overlayer include polyvinyl cinnamate, phenoxy cinnamate, cinnamoylated cellulose, cinnamoylated allyl alcohol polymers, cinnamoylated alkoxylated novolacs, cinnamoylated hydroxyalkyl acrylate polymers, and cinnamoylated hydroxyalkyl methacrylate polymers. Specific examples of other such cinnamoylated resins and cinnamate esters are described in U.S. Pat. No. 3,808,004, No. 3,804,628, and No. 3,387,976. Especially preferred cinnamate esters are the phenoxy cinnamate polymers.
The acrylated resins which can be employed in the photochromic compound/photopolymer layer of photoreactive compositions embodying the present invention can be generally described as oligomers and polymers having recurring pendant groups corresponding to the following structural formula: ##STR2## wherein "X" is S, O or N-R with R being H or lower alkyl, and R.sub.1 and R.sub.2 being the same or different and being selected from the class H and lower alkyl of from 1 to 12 carbon atoms. Preferably, "X" is O, R.sub.1 is H or methyl, and R.sub.2 is H.
Illustrative examples of acrylated photopolymer resins which can be employed with the photochromic compound in the overlayer include the acrylated or methacrylated: polyvinyl alcohol polymers, phenoxy resins, styrene-allyl alcohol copolymers, cellulose, starches, inositol, hydroxyalkyl acrylate polymers, novolac resins, alkoxylated novolac resins, polyvinyl amine polymers, epoxy resins, polythiols, polyvinylhydroxy aryl polymers, alkoxylated polyvinylhydroxy aryl polymers, allylamine polymers, and imine polymers (e.g. polyethylene imine, polypropyleneimine and deacetylated chitin). The acrylic ester polymers are preferred with the acrylated phenoxy resins such as acrylated bisphenol A-epichlorohydrin thermoplastic polymer the acrylated epoxy novolacs and the acrylated styrene-allyl alcohol polymers being found to be particularly desirable for use with indolinobenzospiropyran compounds in the overlayer on the diazo resin coating in the photoreactive compositions of the present invention.
It will be appreciated that the photochromic compound/photopolymer overlayer can also include other components such as flow modifying agents, plasticizers, surfactants, fillers, pigments, dyes, antihalation agents, sensitizers and the like to impart other desired properties thereto.